1. Field of the Invention
The present invention relates to image forming apparatuses that control image forming conditions based on a result of detecting a charge amount of toner.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image upon a photosensitive member based on an image of a document read by a reader, transferred from an external PC, or the like, and forms a toner image by developing the electrostatic latent image on the photosensitive member using toner in a developer. The image forming apparatus controls the density of the toner image by controlling image forming conditions such as an exposure amount of laser light emitted from an exposure apparatus for forming the electrostatic latent image on the photosensitive member, a developing bias for developing the electrostatic latent image on the photosensitive member, a charging potential for charging the photosensitive member, and so on. However, a charge amount of the toner in the developer changes when the toner in the developer is consumed and the developer is refilled with new toner during the formation of many toner images. The charge amount of the toner in the developer also changes in response to changes in the temperature, humidity, and so on within the developer. It is desirable to control the image forming conditions in accordance with the charge amount of the toner in the developer in order to control the density, color, and so on of the toner image in a precise manner.
U.S. Pat. No. 5,006,897 discloses an apparatus including a probe that recovers a small amount of toner from a magnetic brush roller in a developer and measures a charge amount of the toner in the developer based on a mass of the toner recovered by the probe and a change in the amount of electric charge on the magnetic brush roller. According to U.S. Pat. No. 5,006,897, first, the probe, which includes a piezoelectric crystal resonator and an electrode, is caused to attract toner located upon the magnetic brush roller of the developer, and the piezoelectric crystal resonator is then caused to vibrate. A mass M of the toner that adheres to the probe is then calculated based on a difference between a vibration frequency when the toner adheres to the probe and a vibration frequency when no toner adheres to the probe. Furthermore, because toner moves from the magnetic brush roller to the probe, an amount of electric charge Q of the toner adhering to the probe can be found by measuring a change in the amount of electric charge on the magnetic brush roller. The charge amount of the toner in the developer can then be detected based on the mass M and the amount of electric charge Q of the toner adhering to the probe.
However, according to U.S. Pat. No. 5,006,897, the probe is caused to attract toner remaining on the magnetic brush roller after the electrostatic latent image on the photosensitive member has been developed (called “residual toner” hereinafter), and the charge amount of the residual toner is then detected. In other words, according to U.S. Pat. No. 5,006,897, the charge amount of the toner adhering to the photosensitive member is different from the charge amount of the residual toner detected by the probe, and thus there is a problem in that the image forming conditions for forming a toner image at a desired density cannot be set in a precise manner. According to U.S. Pat. No. 5,006,897, even if, for example, the charge amount of the toner has changed drastically, the toner image will be formed based on image forming conditions set before the change in the charge amount of the toner. It is further possible that the charge amount of the residual toner on the magnetic brush roller after the toner has been caused to adhere to the electrostatic latent image on the photosensitive member will have a different value than the charge amount of the toner caused to adhere to the photosensitive member.
FIG. 4 illustrates differences in the amounts of toner adhering to an electrostatic latent image upon a photosensitive member in the case where there are different charge amounts for the toner. The following will describe a case in which the image forming conditions are set based on the charge amount of the toner detected at a predetermined timing. Accordingly, in the following descriptions, a target value for the charge amount is equal to the charge amount of the toner detected at a predetermined timing. Furthermore, in the following descriptions, the amount of toner adhering to the electrostatic latent image on the photosensitive member in the case where the charge amount of the toner is the target value corresponds to a target amount of the toner, at which an image can be formed at a desired density. In the case where the charge amount of the toner used for developing is greater than the target value, the amount of toner adhering to the electrostatic latent image on the photosensitive member will be lower than the target amount. An image that is lighter than the desired density will be formed as a result. On the other hand, in the case where the charge amount of the toner used for developing is lower than the target value, the amount of toner adhering to the electrostatic latent image on the photosensitive member will be greater than the target amount. An image that is darker than the desired density will be formed as a result. Note that in FIG. 4, the vertical axis represents a surface potential of the photosensitive member, Vl represents a light potential (a potential at a region of the photosensitive member that has been exposed), Vcont represents a developing contrast potential difference, Vdev represents a developing bias, Vd represents a dark potential (a potential at a region of the photosensitive member that has not been exposed), and Vback represents a potential difference between the dark potential and the developing bias.